The proposed experiments are designed to test the hypothesis that inputs from different classes of submodality-specific mechanoreceptors are spatially segregated in the dorsal columns-medial lemniscal system. The research objectives are: 1) to determine if inputs to neurons in the ventroposterior inferior nucleus (VPI) of the thalamus are dominated by Pacinian afferents, and 2) to determine the organization of connections between VPI and Pacinian-input zones in cortical somatosensory area II (SII) and the dorsal column nuclei. In the electrophysiological experiments, we will use a battery of manual and mechanical tests to distinguish inputs from different classes of mechanoreceptors. Functional borders will be delineated and the connections investigated by making combined iontophoretic injections of horseradish peroxidast (HRP) and tritiated amino acids (3H) in VPI or an injection of HRP in VPI and 3H in SII. The proposed studies will enhance our understanding of the fine details in the neural processing of certain tactile stimuli and contribute toward the development of more effective clinical tests to detect sensory loss due to a focal injury or lesion in the somatosensory pathway.